1. Field of the Invention
The present invention relates to a semiconductor device having a via plug formed from Cu.
2. Description of the Related Art
In order to accelerate an LSI operation, an insulating film with a low dielectric constant is used as an interlayer insulating film. This insulating film is formed by lowering a density of a material for the film or eliminating polarity in the material. For example, a porous material is generally produced in order to lower a material density. As a result of lowering a film density in order to lower a relative dielectric constant, there is a tendency that the Young's modulus decreases with a decrease in relative dielectric constant, thus the linear expansivity increases. In particular, in recent years, in a low dielectric insulating film with a relative dielectric constant of 3.0 or less used to reduce a low dielectric constant, in general, the Young's modulus has been as low as 10 GPa or less, and the linear expansivity has been greater than 30×10−6° C.−1.
In a high temperature process such as annealing or sintering, with an electrically conductive material such as a barrier metal or an interlayer insulating film, a thermal stress is generated due to a linear expansivity difference from materials which form interfaces, respectively. A linear expansivity of Cu which is a wiring material is as great as about 16 to 30×10−6° C.−1 in the temperature range from room temperature to 500° C. However, a barrier metal layer used between Cu and an interlayer insulating film includes a high melting point metal such as Ta or Ti or its component, with a linear expansivity is 10×10−6° C.−1 or less.
Therefore, in the case where a low dielectric constant insulating film has been used as an interlayer insulating film, a high concentration of a great tensile stress occurs with a barrier metal layer which is smaller than both of Cu and the insulating film with a low dielectric constant in linear expansivity. Since it is sandwiched between Cu having a comparatively high linear expansivity and the insulating film with a low dielectric constant. A thermal stress generated with this barrier metal increases with an increase in linear expansivity of a low dielectric constant insulating film disposed in the periphery of an electrically conducting layer such as a wire or a via plug. In particular, in the case where the linear expansivity of the insulating film with a low dielectric constant is greater than that of Cu (30×10−6° C.−1), a thermal stress applied to the barrier metal layer rapidly increases.
At a side wall portion of the via plug, film thickness of the barrier metal is thinner than that of another portion. Thus, there is a possibility that a crack occurs because of a thermal stress which affects a barrier metal layer. In general, a low dielectric constant insulating film has a low breakage strength, and the crack generated with the barrier metal layer may be advanced up to the inside of the low dielectric constant insulating film. As a result, with respect to an electrically conductive material such as Cu which is in a compression stress state at a high temperature, a short-circuit failure may occur due to the breakage caused by a crack (Jpn. Pat. Appln. KOKAI Publication No. 2003-197742).